1. Field of the Invention
The present invention relates to adjacent channel rejectors for communication systems.
2. Description of the Prior Art
A common problem in radio communication systems such as TACAN and which employs pulse modulation is that there can be interference in the spectrum of signals emitted on a frequency which is near or adjacent to the channel of interest. An example of this problem is illustrated with reference to FIG. 1 which shows a typical spectrum of energy radiated by a pulse transmitter. As shown, the pulse signal on a channel frequency f2 necessarily introduces some energy into the channels f1 and f3. Any receiver attempting to determine whether a desired signal is being transmitted on its channel, say f3, will therefore receive energy from a signal present on channel f2. There have been some attempts in the past to make receiver channels very selective, but as shown in FIG. 1, this does not reject all the energy from the adjacent channel. Ordinarily, the desired signal on the channel, say f3, will be stronger than the residual energy emanated from a pulse signal on the frequency channel f2 and no detrimental effects will be noticed. Occasionally, however, the strength of the undesired signal, say on channel f2, is much stronger in portions of the channel near the cut-off position than the desired signal. This may happen when trying to receive a signal from a distant transmitter while physically located near a transmitter operating on an adjacent channel. In this case, the residual energy from the undesired signal may be strong enough to cause unacceptable interference with the desired signal. A common type of adjacent channel rejection system used in TACAN, is known as a Ferris discriminator. A Ferris discriminator generally operates in an analog mode and makes comparisons to provide the requisite adjacent channel rejector. A typical Ferris discriminator consists of high and low "Q" input tank circuits, two detecting diodes, and r-f bypass and voltage divider loading networks, two clamping diodes, and two triode amplifiers. The Ferris discriminator receives its input to detect the i-f signal and provides adjacent channel and near-adjacent channel rejection.